Lateral stability of the proximal interphalangeal joint.

Current diagnostic criteria and therapeutic guidelines for injuries to the collateral ligaments of the proximal interphalangeal (PIP) joint are imprecise and vague. Laxity determinations, failure analysis, radiographic stress testing, and microscopic dissections were performed on 112 PIP joints. The lateral collateral ligament (LCL) is the primary restraint to varus and valgus angulation of the PIP joint. Its palmar fibers are tight in joint extension and provide the first line of resistance to lateral angulation. Failure of the LCL almost always occurs proximally in a sequential fashion that begins with the palmar fibers and progresses to the more dorsal bundles. Proximal LCL disruption is followed by separation of the accessory collateral-LCL junction and finally by failure of the distal palmar plate. Midsubstance tears of the LCL are rare. If the lateral stress test shows more than 20 degrees of varus or valgus angulation, the LCL can be presumed to be completely disrupted. Angulation of less than 20 degrees is associated with a 53% chance of partial LCL failure and a 47% chance of complete disruption, but the proper position of the LCL will be maintained by the overlying connective tissues. A clinical investigation will be necessary to define the criteria for surgical intervention.